mm, slab: remove unnecessary unlikely()

WARN_ON() already contains an unlikely(), so it's not necessary to use
unlikely.

Also change WARN_ON() back to WARN_ON_ONCE() to avoid potentially
spamming dmesg with user-triggerable large allocations.

[akpm@linux-foundation.org: s/WARN_ON/WARN_ON_ONCE/, per Vlastimil]
Link: http://lkml.kernel.org/r/20181104125028.3572-1-tiny.windzz@gmail.com
Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/slab_common.c

@@ -1029,10 +1029,8 @@ struct kmem_cache *kmalloc_slab(size_t size, gfp_t flags)
 
 		index = size_index[size_index_elem(size)];
 	} else {
-		if (unlikely(size > KMALLOC_MAX_CACHE_SIZE)) {
-			WARN_ON(1);
+		if (WARN_ON_ONCE(size > KMALLOC_MAX_CACHE_SIZE))
 			return NULL;
-		}
 		index = fls(size - 1);
 	}